A prior art system 10 for interrogating one or more radio frequency transponding modules 12 is described with reference to FIG. 1. The system 10 includes an interrogator 13 operating in response to commands from a controller 14. Data and commands are exchanged between the interrogator 13 and the controller 14 through interconnections 15.
In one mode of operation, a transmitter TX 16 contained in the interrogator 14 supplies RF signals through interconnections 17 to a transmit/receive (T/R) antenna system 18. The T/R antenna system 18 in turn radiates an interrogation signal 20 to one or more of the transponding modules 12. When the interrogation signal 20 is received by one of the transponding modules 12, a response signal 24 may be generated and transmitted. The response signal 24 typically includes modulation allowing some property or set of properties of the transponding module 12 to be determined.
The response signal 24 is received by the antenna system 18 and is coupled to a receiver RX 28. The receiver RX 28 demodulates the received response signal 24 and supplies information determined from the received response signal 24 to the controller 14 via the interconnections 15. The controller 14, in turn, may be able to supply information derived from the response signal 24 to an external processor (not illustrated) via a bus or other data link 30.
Systems such as the system 10 find wide-ranging application in a broad variety of settings. The IPASS toll collection system presently in use in the Chicago area is one example of such a system. In transponder-based toll collection systems, the presence of an object, such as a moving vehicle (not illustrated), is detected by a detector. In a toll collection system, the detection process may rely on reflection of electromagnetic waves, detection of magnetic anomalies or detection of a large mass.
In response to detection of the presence of the vehicle, the controller 14 causes the transmitter TX 16 to transmit interrogation signals 20 having carefully limited range. The transponding module 12 is typically a dash-mounted unit that has been pre-programmed to identify the vehicle and to provide information regarding a pre-existing account associated with that vehicle.
The transponding module 12 transmits a response signal 24 containing information to the interrogator 13. The information in the response signal 24 identifies the vehicle and the account associated with that transponding module 12. The receiver RX 28 receives the response signal 24, demodulates this information and couples the received information to the controller 14. The controller 14 may pass the received information to one or more computers (not shown) via the data link 30, causing the account associated with the vehicle to be appropriately decremented. As a result, the vehicle need not slow for the toll collection process, providing smoother traffic flow, reducing fuel consumption and generally facilitating both vehicular transportation and toll collection.
Similarly, an automated fuel retail system (not illustrated) may use the system 10 including the interrogator 13. The interrogator 13 exchanges signals with the transponder 12 that is attached to vehicle (not illustrated) to determine whom to bill for fuel dispensed to that vehicle when the vehicle is fueled at the fuel dispensing station.
In these kinds of systems 10, a range of the interrogation signal 20 is carefully limited to avoid interrogation of more than one transponding module 12 at a time. Additionally, it is often desirable to limit the amount of power that is required by the transponding module 12 in order to provide a stand-alone transponding module 12 that is able to operate without requiring an external source of power and that has long battery life.
Further, it is generally desirable to provide systems 10 that are as simple as possible. One approach to realizing these goals is to receive and transmit the interrogation 20 and response 24 signals in a common frequency band. In some systems, the response signals 24 are derived from the interrogation signals 20 and the transponding module 12 may even be one that simply modulates and re-transmits the received interrogation signal 20 to provide the response signal 24.
One simple technique for modulating the interrogation signal 20 to form the response signal 24 is to either (i) forward bias a diode that is coupled across an antenna in the transponding module 12, corresponding to a first logical state, or (ii) to leave the diode unbiased or reverse biased, corresponding to a second logical state. The transponding module 12 repeatedly switches between these two logical states according to predetermined patterns while the interrogation signal 20 is present to modulate the response signal 24 with a binary signal. These kinds of systems 10 are known as backscatter systems.
In some backscatter systems, the interrogation signal 20 includes a modulated preamble that carries data identifying which transponding module 12 in a population of such transponding modules 12 is targeted to respond. The preamble of the interrogation signal 20 further may include a request for specific data from the targeted transponding module 12. In such backscatter systems, the interrogation 20 and reply 24 signals must necessarily use the same frequency band and both the interrogation 20 and response 24 signals are present at the same time.
Systems 10 that simultaneously or contemporaneously transmit interrogation signals 20 and receive response signals 24 having common frequencies using a common antenna or closely spaced transmit and receive antennas for the T/R antenna system 18 often include some sort of component for separating the transmitted interrogation 20 and received response 24 signals. These systems 10 must particularly ensure that the transmitted interrogation signal 20 does not feed directly into the receiver RX 28.
For example, circulators (not illustrated) are employed in some types of systems where a single antenna is used for both transmission and reception of signals in a common frequency band. Circulators typically have three or more ports, and have the property that each port is strongly coupled to one of the two adjacent ports (referred to as forward coupling) but is not strongly coupled to the other of the two adjacent ports (referred to as reverse isolation).
However, circulators tend to be somewhat bulky, include a large permanent magnet and provide limited reverse isolation. For example, an antenna that is matched to provide a VSWR (voltage standing wave ratio) of 1.5:1 will provide a 20 dB return loss. When portions of transmitted signals are reflected from the antenna back into the circulator, an unacceptably large amount of RF energy may be coupled back into the receiver RX 28 from the transmitter TX 16.
Another approach for reducing unwanted coupling between the transmitter TX 16 and the receiver RX 28 is to employ separate transmitter and receiver antennas (not illustrated) in the antenna system 18. The receiver antenna is placed in or near a null in a radiation pattern associated with the transmitter antenna. Typically, the receiver antenna is placed somewhat behind the transmitter antenna.
One problem with this approach is that an object that reflects substantial portions of the interrogation signal 20 may pass through the radiation pattern of the transmitter antenna such that a large reflected interrogation signal 20 impinges on the receiver antenna. When such reflections occur, large undesired signals may be introduced into the receiver RX 28, causing the received RX 28 to fail to respond to weaker but desired signals.
Similar kinds of systems 10 are presently of great interest for identifying, sorting, counting and routing in situations where selected objects in a population of objects require individual recognition and treatment. Examples include luggage-handling and routing systems associated with public or private transportation systems, package handling and routing systems, vehicle or other rental or check-out systems and inventory control systems.
Some kinds of systems 10 may interrogate a large number of transponding modules 12 simultaneously. For example, an inventory control system may be used to determine if a specific item coupled to the target transponding module 12 is contained in a warehouse. Typically, each transponding module 12 is associated with an inventory item in the warehouse and vice versa.
In these types of systems, code division multiple access may be used to discriminate between responses from multiple transponding modules 12. Alternatively, a preamble including a code or serial number unique to the desired target transponding module 12 may be transmitted by the interrogator 13, and only the target transponding module 12 responds to the interrogation signal 20.
Other schemata include (i) transmitting interrogation signals 20 from the interrogator 13 to a group of responding target transponding modules 12, (ii) distinguishing some response signals 24 from the group of target transponding modules 12, (iii) transmitting signals from the interrogator 13 to turn those transponding modules 12 identified from the response signals 24 OFF, (iv) iterating steps (i)-(iii) until the desired target transponding module 12 has been identified and interrogated and then (v) transmitting signals from the interrogator 13 to restore the ensemble of transponding modules 12 to their initial status or any other desired status. Other methods for selecting one or more target transponding modules 12 in a population of transponding modules is known as well.
In all of these systems 10, it is generally desirable to reduce the complexity of the transponding modules 12 as much as is feasible without compromising the functions that the transponding modules 12 are intended to accomplish. One reason for this is that the system 10 may include a large number of transponding modules 12. Backscatter systems tend to employ very simple and compact transponding modules 12, but tend to have difficulty in ensuring that the interrogation signal 20 does not compromise performance of the receiver RX 28. This is particularly troublesome in backscatter systems because both signals are present in the same frequency band at the same time and because the transmitter and receiver antennas must be physically close to each other. Accordingly, it is helpful to reduce the amount of the interrogation signal 20 that impinges on the receiver RX 28.